Metal containers, such as aluminum beverage and aerosol containers, are typically formed by impact extrusion or by cupper/bodymaker methods. Impact extruded metal containers are formed by plastic deformation of a disk-shaped metal slug into a cylindrical container having approximately the same height, diameter, base thickness and wall thickness as the finished container. The metal slug is placed at the bottom of a cylindrical die and struck with a high-speed cylindrical punch. The impact causes the metal slug to flow backward along the punch to form the extruded cylindrical container.
The extruded container is wall ironed to the diameter and wall thickness of the finished container, by placing the container over a cylindrical ironing punch and passing the container through a ring of narrowing diameter. The reduction in diameter and thickness of the wall causes the container to increase in length. The wall ironed container is then trimmed to the appropriate height.
The trimmed, wall ironed container may then receive interior and exterior coatings, such as primers, lithography and lacquer. The top of the container may also be shaped to form a neck, by insertion into a series of neck forming dies, and then threaded, curled or otherwise shaped to receive a screw cap, aerosol nozzle or other closure. Further shaping operations may be applied to the body of the container, to form a grip or other design.
Because of the amount of work required to plastically deform the metal slug, it has been necessary to manufacture the containers using relatively soft metal alloys, such as 1000 series aluminum which has less than or equal to 1% impurities. The use of such soft aluminum alloys requires the container to be designed with a relatively thick wall and base, to provide sufficient strength when the containers are stacked or when the contents are pressurized. High strength alloys, such as 3000 series aluminum alloys, would permit the manufacture of relatively lightweight containers with significantly thinner walls and base, while providing sufficient strength to withstand the weight of stacked containers or internal pressurization. However, such high-strength alloys are difficult to form by impact extrusion, and cause excessive wear and replacement of extrusion tooling. Thus, it has not been economically feasible to produce metal containers using high-strength alloys by impact extrusion.
Thin-walled containers made of high-strength alloys are typically produced from coiled metal sheet stock using the cupper/bodymaker method. The thickness of the metal sheet is preselected to be the same as the base thickness of the finished container, thus avoiding the severe deformation of the metal required to form the container by the impact extrusion process. The metal coil is unwound and fed into a cupper, which stamps a round blank from the sheet. The blank is then pressed into a die to form a cup-shaped cylindrical container, that has a substantially larger diameter and is correspondingly shorter than the finished container. Because the metal is not plastically deformed, the base thickness and wall thickness of the cup retains the thickness of the metal sheet stock.
The cup is transferred to a bodymaker, which performs a series of wall ironing operations to sequentially reduce the diameter and wall thickness, and increase the height of the container to its appropriate height, diameter and wall thickness. The wall ironed container is then trimmed, necked and finished as described above for impact extruded containers.
There are several drawbacks to using the cupper/bodymaker method in comparison to impact extrusion methods. In particular, additional space and equipment is required for storage and handling of the large, heavy metal coils used to produce the containers. Furthermore, the cupper/bodymaker equipment is specifically designed to produce containers having a particular diameter and height, and cannot efficiently be adapted to produce alternate size containers. Thus, each size container typically requires a separate cupper/bodymaker and production line.
In contrast, the metal slugs used in the impact extrusion method do not require special handling or storage and the extrusion equipment is readily adapted to produce different sized containers by simply changing the size of the metal slug, and/or the size of the extrusion die and punch. Furthermore, the thickness of the base of the container can be changed by controlling the force of the extrusion punch, whereas the cupper/bodymaker method is limited to producing containers having the same base thickness as the thickness of the metal sheet stock.
In addition, the cupper/bodymaker method uses materials less efficiently than the impact extrusion method. Once the blanks are stamped from the metal sheet stock by the cupper, the exhausted metal sheet must be recycled or scrapped. Thus, a significant portion of the material cost is not incorporated into the containers. Such costs are avoided by the impact extrusion method, which uses preformed metal slugs as the starting material. Furthermore, metal slugs are available in a broad range of sizes and alloys, and can be purchased in relatively small numbers from a wide range of suppliers, which allows production to be flexibly switched between small lots of different types of containers. In contrast, the metal coils used in the cupper/bodymaker method are only available in bulk quantities from a few suppliers, which restricts production to relatively large numbers of a single type of container.
Thus, there is a need for a method of producing metal containers that permits the use of high-strength metal alloys and that can readily be adapted to produce containers of different height and diameter.